1. Field of the Invention
The invention relates to a control apparatus for an internal combustion engine that is applied to an internal combustion engine that can change over a combustion mode between lean combustion and stoichiometric combustion.
2. Description of Related Art
There is known a lean burn engine in which lean combustion can be carried out with a target air-fuel ratio set on a leaner side than a theoretical air-fuel ratio. In the lean burn engine, stoichiometric combustion is carried out at the time of acceleration, at the time of low rotation and low load, etc. With stoichiometric combustion, the internal combustion engine carries out combustion at the theoretical air-fuel ratio, which is on a richer side than the target air-fuel ratio of lean combustion, or at a target air-fuel ratio in the vicinity thereof. The internal combustion engine makes a changeover to stoichiometric combustion because a sufficient engine torque cannot be ensured by lean combustion at the time of acceleration, and because the temperature of a catalyst is likely to fall at the time of low rotation and low load.
As a control apparatus applied to this internal combustion engine, there is known an apparatus that performs the following procedure when the combustion mode needs to be changed over from stoichiometric combustion to lean combustion (Japanese Patent Application Publication No. 2008-121511 (JP 2008-121511 A)). When a request to change over the combustion mode from stoichiometric combustion to lean combustion is made, this control apparatus increases the amount of intake air by increasing the opening degree of a throttle, increases the amount of fuel injection, and restrains the torque of the engine from increasing as a result of these increases by retarding an ignition timing. Then, the control apparatus changes over the combustion mode from stoichiometric combustion to lean combustion by advancing the ignition timing after the lapse of a predetermined time, controlling the ignition timing in such a manner as to correspond to an operation state, and reducing the amount of fuel injection. Besides, Japanese Patent Application Publication No. 2000-161098 (JP 2000-161098 A) is regarded as an art related to the invention.
The control apparatus of Japanese Patent Application Publication No. 2008-121511 (JP 2008-121511 A) keeps the air-fuel ratio from remaining equal to an intermediate air-fuel ratio, which is between a target air-fuel ratio of stoichiometric combustion and a target air-fuel ratio of lean combustion, by changing the air-fuel ratio in a stepped manner, in consideration of the fact that the exhaust emission properties deteriorate when the air-fuel ratio remains equal to the intermediate air-fuel ratio during changeover of the combustion mode. In order to change the air-fuel ratio in this stepped manner, the control apparatus controls the amount of intake air to an intake air amount that is needed for the target air-fuel ratio after changeover of the combustion mode, and then changes the amount of fuel injection in a short time. A fluctuation in engine torque resulting from the control of the intake air amount is suppressed through the control of the ignition timing.